Removal of hydrogen sulfide from a fluid stream by a liquid phase oxidation is known to the industry. See, for example, U.S. Pat. Nos. 3,097,925; 3,676,356; 3,897,219; 3,975,508; 4,009,251; 4,011,304; 4,014,983; 4,036,942; 4,076,621; 4,189,462; 4,218,342; 4,238,462; 4,374,104; 4,455,287; 4,482,524; 4,622,212; 4,705,676; and British Patent No. 948,270; the disclosures of which patents are incorporated herein by reference. U.S. Pat. No. 4,374,104 is of particular interest in showing a composition containing chelated metal catalysts and surfactants of the generic type used in the present invention. The liquid phase oxidation process, in general, comprises bringing a hydrogen sulfide gas-containing stream into intimate contact with an aqueous oxidizing reaction solution. The reaction solution preferably comprises an aqueous oxidizing solution containing polyvalent metal ions (M) as a catalyst which receives electrons from the sulfide ion in aqueous solution to form elemental sulfur while reducing the metal ions from their higher valence state to a lower valence state, as illustrated by the following equation: EQU S.sup.-2 +2M.sup.+3 =S.sup.O +2M.sup.+2
In order to regenerate the catalytic metal ions to their original higher valence state, the reduced reaction solution is contacted with oxygen dissolved in the aqueous reaction solution, as illustrated by the following equation: EQU 1/2O.sub.2 +2M.sup.+2 +H.sub.2 O=2(OH).sup.- +2M.sup.+3
Among the polyvalent metals which can be used in the aqueous reaction solution in ionic form are iron, copper, vanadium, manganese, platinum, tungsten, nickel, mercury, lead, and tin, with iron being preferred.
In order to provide an economical, workable, continuous process for removing hydrogen sulfide gas from a fluid stream in which polyvalent metal ions are used to effect catalytic oxidation of hydrogen sulfide, it is desirable to provide an aqueous catalytic reaction solution which is (a) stable at slightly acidic and alkaline pH values over a relatively wide pH range (preferably from about pH 5.5 to pH 13), and which is (b) capable of being rapidly regenerated after effecting oxidation of the hydrogen sulfide. This regeneration step (b), should occur without significant loss of the catalytic metal ions and/or sulfur and it should occur without an objectionable buildup of elemental sulfur in the reaction solution. While certain aqueous oxidizing reaction solutions containing a polyvalent metal ion catalyst, such as vanadium in the Stretford process, are relatively stable, other aqueous reaction solutions which contain a metal catalyst, such as iron, must be stabilized against precipitation of metal hydroxides and metal sulfides. Such stabilization is achieved by including in the reaction solution a chelating agent which maintains the metal catalyst in solution in both its higher and lower valence states in alkaline solution, and preferably over the pH range from about 5.5 to about 13. The continuous oxidation of hydrogen sulfide to form elemental sulfur by a chelated aqueous catalytic metal oxidation-reduction reaction solution and the regeneration of the reduced aqueous catalytic metal reaction solution can be represented by the following equations:
(1) Oxidation-Reduction: EQU H.sub.2 S(gas)+2(M chelate).sup.+3 2H.sup.+ +S.sup.o +2(M chelate).sup.+2
(2) Regeneration: EQU 1/2O2(gas)+2(M chelate).sup.+2 =2(OH).sup.- +2(M chelate).sup.+3
It is generally desirable to maintain a minimum concentration of between about 0.1 and about 10 weight percent sulfur in the reaction solution in order to increase the overall size of the sulfur particles in the reaction solution. A sulfur concentration in excess of about 10 weight percent can result in plugging of equipment and also interfere with the catalytic reactions. The most economical means for reducing the concentration of elemental sulfur in the reaction solution is to cause the sulfur to settle out of the reaction solution. However, because the elemental sulfur is formed while rapidly mixing a large volume of hydrogen sulfide-containing gas with a large body of aqueous reaction solution, a gas-liquid-solid sulfur dispersion is formed as a result of a large volume of gas moving at a relatively high velocity relative to a liquid reaction solution, and extremely fine solid elemental sulfur particles are formed in the reaction solution. Such fine particles have surface properties which cause the particles to adhere to minute gas bubbles in an aqueous reaction solution and float to the surface where they form a thick layer of froth on the surface of the reaction solution. This prevents rapid and complete settling of the sulfur and it increases the difficulty f separating and recovering the sulfur from the reaction solution. Treated gas compressor suction manifolds and screens tend to plug with fine sulfur particles requiring troublesome and time consuming cleaning and maintenance. Absorber internals also tend to become clogged with sulfur resulting in increased gas pressure drops. This also requires frequent plant shutdowns for cleaning.
It is, therefore, an object of the present invention to provide an improved aqueous liquid phase oxidizing solution from which elemental sulfur formed therein can be more efficiently removed.
It is also an object of the present invention to provide a substantially greater concentration of sulfur in the solution withdrawn from the reaction zone than has heretofore been achieved.
It is a further object of the present invention to provide an improved aqueous metallic chelated oxidation-reduction reaction solution adapted for liquid-phase oxidation of hydrogen sulfide gas to form elemental sulfur, wherein the sulfur does not remain suspended in the reaction solution for a prolonged period of time or float to the surface and form a layer of froth on the surface of the reaction solution.
It is still another object of the present invention to provide a more economical process for the removal of hydrogen sulfide gas from a fluid stream and recovery of elemental sulfur from a catalytic liquid-phase oxidizing aqueous solution.
Other objects of the present invention will be apparent from the accompanying detailed description and claims to follow.